The present invention relates to a nonwoven having a filament density pattern yielding low tensile strength and high percent elongation in a first direction (e.g., CD) and high tensile strength and low percent elongation in a second direction (e.g., MD), and a method of making such a nonwoven.
Spunbond nonwoven fabrics are made of continuous strands or filaments that are laid down on a moving conveyor belt in a randomized distribution. In a typical spunbond process, resin pellets are processed under heat to a melt and then fed through a spinnerette to create hundreds of thin filaments or threads. Jets of fluid (e.g., a gas such as air) causes the threads to be elongated, and the threads are then blown or carried onto a moving web where they are laid down and sucked against the web in a random pattern to create a fabric. The filament density of the fabric is typically fairly uniform and symmetrically extending in both the machine direction (MD) and the cross direction (CD) (at least this is the goal). The fabric then passes through a bonding station. Bonding is necessary because the filaments or threads are not woven together.
Other nonwoven fabrics include fabrics made from staple fibers, meltblown fibers, carded fibers and short cut fibers. As used herein the terms xe2x80x9cspun fibersxe2x80x9d and xe2x80x9cfibersxe2x80x9d include the spun filaments described above as well as staple fibers, meltblown fibers, carded fibers and short cut fibers.
A non-uniform and non-symmetrical fiber density pattern could result in a fabric with an expected elongation in one direction (e.g., MD), but an unexpected increased elongation in the other direction (e.g., CD). This is desired in situations where minimal elongation is required in the machine direction and yet high elongation is required in the cross direction. For example, any elongation in the machine direction will disrupt the converting machines that are used to make diapers, while substantial elongation in the cross direction is desired to give each individual diaper some give around the wearer""s waist.
Accordingly, it is an object of the present invention to provide a nonwoven fabric having a non-uniform fiber density pattern and a method of making such fabric.
Another object of the present invention is to provide a nonwoven fabric and method of making same wherein in one embodiment the fabric is highly extensible in a first direction (typically the CD) as compared to a second direction (typically the MD).
Another object of the present invention is to provide a nonwoven fabric and method of making same wherein in one embodiment the fibers of the fabric are oriented more in the MD than in the CD thereby providing higher tensile strength in the MD than in the CD.
Another object is to provide such a method and fabric wherein in one embodiment the nonwoven fabric has low tensile strength and high percent elongation in the first direction (typically the CD), and high tensile strength and low percent elongation in the second direction (typically the MD).
It is a further object to provide such a method and fabric wherein in one embodiment the density of the fibers in the fabric varies between strips of relatively high density and strips of relatively low density, the strips extending substantially continuously along the length of the fabric in the machine direction in an alternating pattern.
It is a still further object of the invention to provide such a fabric wherein in one embodiment less fibers are bonded together in the strips of relatively low fiber density than in the strips of relatively high fiber density.
It is a still further object of the invention to provide such a fabric which in one embodiment provides increase liquid wicking in the strips of relatively high fiber density and increased liquid strike-through in the strips of relatively lower fiber density.
It is a still further object of the invention to provide such a fabric which in one embodiment has a shrinkage differential between the strips of relatively high fiber density and the strips of relatively lower fiber density when the fabric is exposed to heat, with greater shrinkage in the strips of relatively high fiber density.
It is also an object of the present invention to provide such a method which in one embodiment is simple and economical to use and maintain.
It is an object of the present invention to provide an improved process for the method of manufacturing a nonwoven according to the present invention and an improved nonwoven wherein, in one embodiment, the design of the fluid impermeable and fluid permeable regions appears regular and a biasing thereof in one direction is difficult to discern by the ordinary viewer.
It is another object to provide such an improved process and nonwoven wherein, in one embodiment, the fabric defines both fluid flow highways of low thickness and parallel fluid flow barriers of high thickness, thereby to encourage fluid flow in the MD and retard fluid flow in the CD.
It is a further object to provide such an improved process and nonwoven wherein, in one embodiment, a composite fabric includes a nonwoven according to the present invention and an elastic film thereon, the composite exhibiting a reduced propensity for breaking of the bonding points upon stretching in the CD.
It is a still further object of the present invention to provide such an improved process wherein, in one embodiment, jets of a liquid fluid (rather than a gaseous fluid) impinge on the spun fibers in order to enhance redirection of the spun fibers.
It has now been found that the above and related objects of the present invention are obtained in a nonwoven fabric formed from spun fibers wherein the density of the fabric varies between strips of relatively high density and strips of relatively low density, the strips extending along the length of the fabric in the machine direction in an alternating pattern. The high density strips are separated from each other by low density strips, the alternating high density and low density strips providing the fabric with a higher percent elongation in the cross direction than in the machine direction. The method comprises the steps of forming a flowing stream of spun fibers and moving an air (or like fluid) permeable member over a suction box and across the path of the fiber stream to intercept and collect the fibers on one face of the air permeable member and to bond the collected fibers together to form a nonwoven fabric. A pattern screen is provided, the pattern screen defining a plurality of elongated air (or like fluid) permeable regions separated by elongated relatively air (or like fluid) impermeable regions, the elongated regions extending in the machine direction in an alternating pattern, with the air permeable regions separated from each other by the relatively air impermeable regions. The pattern screen is positioned in relation to the air permeable member and the suction box to cause the fibers to be collected on the one face of the air permeable member so as to form a fabric having a density that varies between strips of relatively high density and strips of relatively low density. The strips extend along the length of the fabric in the machine direction in an alternating pattern, with the high density strips separated from each other by low density strips, the alternating high density and low density strips providing the fabric with a higher percent elongation in the cross direction than in the machine direction. The collected fibers are then separated from the air pervious member.
In a preferred embodiment, the relatively air impermeable regions disrupt the air flow directly above the air pervious member, and the pattern screen comprises longitudinally extending and transversely spaced air impermeable baffle bars. The pattern screen is preferably below the suction box cover, in the plane of the suction box cover, between the suction box cover and the air permeable member, or part of the air permeable member (i.e., on the top of, on the bottom of or within the air permeable member).
The present invention also encompasses a method of making a nonwoven fabric embodying spun fibers deposited in a patterned formation providing a higher percent elongation in CD than in MD. The method comprises the steps of forming a flowing stream of spun fibers and air (or like fluid), and moving an air pervious member over a suction box cover and across the path of the fiber stream to intercept and collect the fibers on one face of the air pervious member. A pattern screen is disposed as the suction box cover, thereby to cause the fibers to be collected on the one face of the air pervious member in a pattern determined by the pattern on the pattern screen and to bond the collected fibers together to form a nonwoven fabric. The pattern defines an air pervious area/air impervious area ratio which is greater in MD than in CD, thereby to provide a fabric with a higher percent elongation in CD than in MD. The collected fibers are then separated from the air pervious member.
In a preferred embodiment, the pattern screen comprises longitudinally extending and transversely spaced substantially air impermeable baffle bars thereby to impose the pattern of the baffle bars on the suction exerted on the filaments by the suction box. The apertures of the air pervious member define a regular non-oriented pattern.
The present invention further encompasses an improved method of making the nonwoven fabric using a pattern screen wherein the elongated fluid permeable regions are configured and dimensioned as rhomboids elongated in the MD, or wherein a liquid (such as water) redirects the spun fibers in the formed web (with removal of the liquid being performed by a liquid discharge box rather than a suction box), or wherein the thickness of the fabric varies between strips of relatively high thickness (which act as speed bumps to fluid flow in the CD) and strips of relatively low thickness (which act as highways for fluid flow in the MD), or wherein an elastic polymeric film is coated (preferably cast coated) onto the fabric in the MD and the composite of the collected fibers and the elastic film is separated from the fluid permeable member. Preferably the fabric has an MD/CD density ratio of 1.1-10.0 (preferably 1.5-3.0) to 1.0. Preferably the thickness of the low thickness strips is only 10-90% (preferably 25-75%) of the thickness of the high thickness strips.